Tub shaped cartons, i.e. those which are generally cylindrical in shape having one end thereof which is essentially open and is adapted to receive a closure lid thereon, are often formed from a blank comprising a single sheet of paperstock which is folded to form a plurality of overlapping, side wall seams which must be bonded together in order to secure the container in an erected form. Containers of the type mentioned above may also be further provided with a multi-piece, multi-ply reinforcement rim formed integral with the side walls which surrounds the opening in the top of the container.
Selective portions of the paperstock forming the container blank are coated with a layer of a heat meltable plastic which provides the two-fold function of providing a moisture impervious lining inside the container and acting as a bonding medium between overlapping seams of the carton.
As will be discussed more fully hereunder, the prior method for sealing the seams in the side walls and rim of the carton consisted of applying localized heat to such seams in order to melt the plastic material between the overlapping layers of the paperboard, thereby bonding and sealing the same. This prior art method, and associated apparatus for performing the same, are subject to one serious objection. Upon application of localized heat to the seams during the sealing process, the opening surrounding the top of the container, that is, the rim, is permanently deformed into an oval shape. The resulting oval shaped rim is undesirable, of course, both from an esthetic as well as a functional standpoint; for example, difficulty is encountered in fitting the deformed, oval shaped rim with a circularly shaped closure lid.
Accordingly, it is a primary object of the present invention to provide a novel method and related apparatus for heat sealing the seams of a tub shaped carton in a manner which avoids deformation of the rim. This and further objects of the invention will become clear or be made apparent during the course of the following description.
According to the present invention, apparatus for implementing the novel method includes a multi-piece, annularly shaped, heated collar which is shiftable into engagement with the rim of a container and conformingly engages the entire periphery of the latter. Heat is applied evenly along the entire periphery, or length of the rim, thereby heating the entire rim, rather than isolated portions thereof, which equalizes the surface tension of the plastic coating on the rim. Equalization of the surface tension of the plastic coating along the rim eliminates deformation thereof during the heat sealing process to allow consistently repeatable fabrication of a nearly perfectly round rim.